Fuse types class J RK1 T
Class J, RK1 and T fuses are current-limiting protective devices conforming to UL 248 and CSA C22.2 standards, designed to operate in low-voltage distribution circuits up to 600 VAC. The class J fuse, governed by UL 248-8, is characterized by its compact size and high interrupting capacity of 200 kA symmetrical; the class RK1 fuse, under UL 248‑12, incorporates a dual-element fuse link with mechanical rejection that prevents insertion of class H or K cartridges; the class T fuse, specified in UL 248‑15, offers the highest energy density per volume, with a typical breaking capacity of 200 kA and dimensions up to 40 % smaller than an equivalent class J fuse. All three types use a silver or silver alloy fuse element encapsulated in silica sand, ensuring rapid arc extinction under short-circuit conditions.
Technical Characteristics
Section titled “Technical Characteristics”| Parameter | Class J | Class RK1 | Class T |
|---|---|---|---|
| Rated voltage VAC | 600 V / — | 250 V / — and 600 V / — | 300 V / — and 600 V / — |
| Current range | 1 A to 600 A | 0.1 A to 600 A | 1 A to 800 A |
| Interrupting capacity (symmetrical) | 200 kA / — | 200 kA / — (600 V), 300 kA / — (250 V) | 200 kA / — (600 V), 200 kA / — (300 V) |
| Current-limiting class | Current-limiting | Current-limiting, dual-element available | Fast-acting current-limiting |
| Reference standard | UL 248-8, CSA C22.2 No. 248.8 | UL 248-12, CSA C22.2 No. 248.12 | UL 248-15, CSA C22.2 No. 248.15 |
| Operating temperature | –40 °C / –40 °F to 100 °C / 212 °F | –40 °C / –40 °F to 100 °C / 212 °F | –40 °C / –40 °F to 100 °C / 212 °F |
Classification by response time
Section titled “Classification by response time”- Fast-acting. No intentional delay, ideal for resistive loads and circuits without inrush peaks. The pre-arcing I²t constant is low, typically less than 10 000 A²·s for a 100 A cartridge.
- Time-delay. Withstand momentary overloads up to 5 times rated current for 10 s without melting. The class RK1 with dual-element is the main representative; the fuse link contains additional thermal mass that delays melting under motor starting conditions.
- Dual-element. Exclusive to classes RK1 and RK5, integrates a calibrated solder point for short-circuits and a thermal section for overloads. The overload behavior is defined from 135 % of rated current, with a minimum opening time of 1 h.
Construction and dimensional section
Section titled “Construction and dimensional section”Class J fuses use a cylindrical body of melamine or epoxy reinforced with fiberglass, with ferrule terminals up to 60 A and blade terminals for higher currents. Standard dimensions of a 600 A class J fuse are 170 mm / 6.69 in length and 50.8 mm / 2.0 in diameter. RK1 fuses share external dimensions with classes H and K but include a rejection slot in their terminals; lengths vary between 76.2 mm / 3 in for 30 A cartridges and 339.7 mm / 13.37 in for 600 A. Class T fuses reduce volume by up to 40 % compared to an equivalent class J fuse; a 400 A class T cartridge measures approximately 101.6 mm / 4.0 in length and 25.4 mm / 1.0 in diameter.
Applications by class
Section titled “Applications by class”| Class | Main application | Concrete example |
|---|---|---|
| J | Protection of distribution feeders, lighting and power panels, motor control centers (MCC) with limited space | 480 V / 277 V panel for a 5000 m² / 53 820 ft² commercial building |
| RK1 | Protection of motor branch circuits and transformers where selectivity and mechanical rejection are required | 50 HP / 37 kW induction motor starter in a water treatment plant |
| T | Protection in ultra-compact enclosures, compact disconnects, and high-density busbars | 1200 A main disconnect in a data center with 300 kW power modules |
Trip curve and let-through energy
Section titled “Trip curve and let-through energy”The trip characteristic of current-limiting fuses is defined by the Joule integral for the fuse element:
I²t = ∫ i² dt where I is the instantaneous current and t the melting time.
| Symbol | Meaning | Unit |
|---|---|---|
| I²t | Let-through energy (Joule integral) | A²·s |
| I | Instantaneous short-circuit current | A |
| t | Melting time (pre‑arc plus arc) | s |
In 480 V three-phase systems with 65 kA available symmetrical fault current, a 200 A class J fuse limits the peak let-through current to less than 18 kA, with a clearing I²t around 15 000 A²·s. A class T fuse of the same rating reduces the let-through energy by approximately an additional 30 % due to its faster operating speed.
Installation and selectivity
Section titled “Installation and selectivity”Class J and T fuses are installed in fuse holders without a rejection mechanism; the cartridge must be selected by qualified personnel according to equipment markings. Class RK1 is housed only in bases with slotted receptacles, a mechanism that physically prevents insertion of class H or K fuses. To achieve ampacity selectivity between fuses, a minimum ratio of 2:1 in rated currents is recommended (e.g., 200 A upstream and 100 A downstream). Selectivity in the short-circuit zone is verified by comparing let-through energy curves; the upstream fuse must have a pre-arcing I²t that exceeds the total clearing I²t of the downstream fuse.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum interrupting rating of a class J fuse at 600 V?
Section titled “What is the maximum interrupting rating of a class J fuse at 600 V?”The maximum interrupting rating of a class J fuse at 600 V is 200 kA symmetrical RMS, equivalent to 200 000 A RMS, sufficient for the most severe fault currents in industrial installations. This interrupting capacity ensures the fuse can clear the fault without fragmentation of the cartridge body.
In what current range are class T fuses available?
Section titled “In what current range are class T fuses available?”Class T fuses cover a current range from 1 A to 800 A, at voltages of 300 V and 600 V. The 300 V version is used in single-phase or three-phase distribution systems at 208 V/240 V, while the 600 V version predominates in industrial 480 V systems.
How long is a 60 A class J fuse?
Section titled “How long is a 60 A class J fuse?”A 60 A class J fuse measures 57.2 mm / 2.25 in length and 20.6 mm / 0.81 in diameter. These cartridges use ferrule terminals suitable for fuse holders with capacities from 0 A to 30 A and 31 A to 60 A.
What operating temperature do RK1 fuses withstand?
Section titled “What operating temperature do RK1 fuses withstand?”Class RK1 fuses withstand a continuous operating temperature from –40 °C / –40 °F to 100 °C / 212 °F without degradation of their trip characteristics. The temperature rise at the terminals must not exceed 75 °C / 167 °F to comply with UL 248 heating tests.
What is the maximum rated current of a class J fuse?
Section titled “What is the maximum rated current of a class J fuse?”The maximum rated current of a class J fuse is 600 A, with a rated voltage of 600 VAC. This cartridge size uses blade terminals with a standard center-to-center spacing of 44.5 mm / 1.75 in for mounting in disconnect bases.
What I²t let-through energy is required to protect a 50 HP motor?
Section titled “What I²t let-through energy is required to protect a 50 HP motor?”For a 50 HP / 37 kW, three-phase, 460 V induction motor with a full-load current of 65 A, a 100 A time-delay class RK1 fuse is recommended, whose maximum clearing I²t under a 100 kA short-circuit is of the order of 8000 A²·s, a value that effectively limits electromagnetic and thermal stresses on the windings.